Screw compressors (screw presses) are used in particular for compressing and/or for pressing liquids out of various bulk materials. The object of this can be, for example, to compress bulk material, for instance to compress waste matter, such as textile waste or paper waste, for the purpose of reducing the volume thereof, to extract liquid from bulk material, for instance to extract oil from oil-containing plant material, or to concentrate bulk material, for instance to press liquid out of material, such as water out of wet bark, where the pressed bark has a higher dry matter content after pressing than it did before pressing, and consequently a higher thermal value on combustion, for heat production for example. The object can also be to use the screw compressor on its own, or the screw compressor included in a system with further devices and/or further screw compressors, to wash the bulk material by means of (repeated) sequential supply of "purer" liquid to the bulk material and removal of the liquid from the bulk material
The internal friction in many bulk materials and the friction between the bulk material and the surfaces of the screw compressor result in an uneven compression of, and/or removal of liquid from, the bulk material in existing screw compressors where the reduction in the volume of the bulk material does not take place in a uniform manner, since, as a result of the internal friction in the bulk material which is being pressed, liquid in the first instance leaves that (surface) area of the bulk material which is exposed to the greatest compressing movement, which results, inter alia, in a low yield of liquid and a low dry matter content of the bulk material and/or entails a high energy consumption. Inappropriate dimensions and designs of conical screw compressors in the prior art also result in a remixing and breaking up of the material inside the screw compressor, which increases the energy consumption and can have a negative effect on the bulk material.
Screw compressors for removal of liquid need to be provided with some form of resistance means for the bulk material at the outlet opening for bulk material in order to prevent re-expansion of compressed bulk material while it is still in contact with the liquid which has been pressed out, which would result in rewetting of the bulk material. Conventional screw compressors therefore have adjustable resistance means such as, for example, flaps which are folded in towards the outlet, or they have resistance means which are mounted on the screw shank and are adjusted towards the outlet by hydraulics. These constructions are delicate, often give rise to leakage of liquid during removal of the liquid, and mean that the screw often has to be mounted at both the inlet end for bulk material and at the outlet end for bulk material.